In wireless transmitters, voltage-controlled oscillator (VCO) pulling by an associated transmitter output or associated phase-locked loop output degrades the transmitter performance. As a result of the pulling, the VCO output will not be a single tone anymore and will have unwanted frequency components. For example, the transmitter pulling causes the VCO frequency to spread around its center, or in a case of pulling by another phase-locked loop (PLL), the VCO output will have unwanted sidebands. The unwanted frequency components can degrade the transmitter performance by violating the emission mask or degrading the error vector magnitude.
As integrated circuit design moves toward aggressive integration, where power amplifiers and/or multiple PLLs are integrated onto a same die as the VCO, the pulling effect on the VCO increases. Some conventional solutions such as, increasing the distance between the VCO from other VCOs and power amplifiers, increasing the VCO current and increasing the PLL bandwidth, come with die area and power consumption penalty.